How does MMR work?

I understand that mmr is a number that is a reflection of your skill level. But what determines how mmr is raised? Me and my friend have been playing alot of ranked together. This is his first season and he almost always plays his games with me.

I started playing last season but didn't play much ranked, I ended up in silver. And this year after placements I got bronze 2, after climbing out to silver 3 I began to duo with my friend who was placed in silver 3 (or 2 im not sure). We both got promoted to gold 5 in the games we have played together. My question is our mmr, my mmr according the op.gg is 1454 and my friend's mmr is 1572.

Im surprised as he is a good player but I know he is not better than me. My guess on why my mmr is so low was because I was placed in silver last season. This is my friends first season playing and I believe I have been playing since All Stars 2014.